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Department of Small Animal Medicine and Surgery, and Comparative Nutrition Laboratory, College of Veterinary Medicine, Texas A&M University, College Station, TX 77843-4474
3 To whom correspondence should be addressed. E-mail: jbauer{at}cvm.tamu.edu.
KEY WORDS: • skin hydration • epidermal water loss • breed effects • dogs
Mammalian skin is a highly dynamic organ that is constantly adapting to changes in its environment. It provides structural, sensory, immunologic, and physiologic functions and contributes an essential barrier function against potential environmental insults. The epidermis of the skin is composed of keratinocytes that undergo a highly organized maturation process that ultimately leads to desquamation. Structural components within these cells utilize dietary lipids such as linoleic acid to maintain a protective barrier. Together they form the stratum corneum, which is the outermost layer of the epidermis. Environmental, physical, and nutritional alterations can modify epidermal structure and function. In addition to keeping environmental insults outside the body, the epidermal barrier also functions to keep water and other important metabolites inside the body. In this way, metabolic processes consonant with health can be maintained.
Techniques to characterize the skin's barrier function include a number of noninvasive methods to measure moisture content and loss through the skin surface (1). In humans, day-to-day variations of these measurements exist (2) that are also likely to occur in dogs. One of these measurements is the determination of skin hydration (HYDR)4 using a method known as corneometry (3). This technique determines the capacitance of the skin due to its behavior as a dielectric medium and assesses a 10–20-µm thickness of the stratum corneum. Although it is a measure of the water content of the skin, it is only an indirect measure of barrier function. Nonetheless, it can be related to the extent of HYDR under various physiologic conditions in response to either injury, metabolic phenomena, or topical therapies. Values reported for human skin evaluations differ depending on the body site that is studied. Such variation may also be expected in dogs, although little information exists regarding normative canine values. Nonetheless, changes in individual or mean HYDR scores in a group of animals may serve as an index of skin health, with higher values typically considered more desirable.
Another method that has received some attention in dog studies is assessment of the integrity of the skin barrier to insensible water loss (4,5). This technique is performed by measuring transepidermal water loss (TEWL) through the epidermal surface. The TEWL value is a measure of the rate of water lost through the skin (in g/h·m2) and is an estimate of the skin's ability to retain moisture. It is an index of the extent of possible damage of the skin's water-barrier function. Because water loss through the skin normally occurs by passive diffusion through the epidermis, higher TEWL values indicate greater water loss and are consistent with increased damage of the barrier function of the stratum corneum such as may occur during irritant exposure, self-excoriation, or atopic dermatitis.
Results of recent work on TEWL and HYDR in dogs have been reported (4,5). One study (4) investigated conditions under which TEWL measurements might be reliably used in Labrador Retrievers. It was found that less sample variance occurred when dogs were trained to stand reasonably still during the entire measurement period. Also, body-site variation existed, and recent hair clipping was shown to affect the value obtained, although no significant effect of age was found (2–7 vs. 8–11 y old). Another study in dogs (5) investigated both TEWL and HYDR measurements using different body sites in five healthy Beagles. Reproducible results were found with the corneometry measurement for HYDR, but TEWL values were less consistent. It was concluded that TEWL measurements were not reproducible enough to be compared between different animals, body sites, or days of assessment. In contrast, although corneometry values varied among dogs and body locations, the findings were reproducible and believed to be sufficiently reliable for additional investigation.
Although studies in dogs using these techniques are limited, both methods are noninvasive and may prove useful in veterinary clinical dermatology and nutrition. This study was conducted to evaluate HYDR and TEWL measurements in two canine breeds and to determine their variability. The measurements were made in dogs fed the same diet over an 11-wk period to mitigate any variation due to diet over time in the two breeds studied.
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MATERIALS AND METHODS |
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RESULTS |
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Measurements were taken indoors within a temperature range of 20–24°C and relative humidity range of 39–51%. Linear regression analyses within these ranges were found not to significantly correlate with either TEWL or HYDR values. The individual dog mean TEWL values ranged from 2.8 to 50.7 g/h·m2, and mean HYDR values ranged from 9.0 to 57.5 units.
A repeated-measures ANOVA model with TEWL and HYDR as dependent variables was analyzed that resulted in a significant breed effect (P < 0.001) for both measurements (Fig. 1). A statistically significant time effect was observed for HYDR measurements with mean values at wk 3 greater than those obtained at either wk 7 or 11 (Fig. 2). The TEWL measurements were not significantly different over time. No breed x time interactions were found.
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Because of the main breed effects observed from ANOVA, the variance and percent CV (%CV) values for Beagles and hounds were calculated separately and compared (Table 1). Within-animal variance for the Beagles was nominally lower but not statistically significant compared with the hounds, and no time effects were observed. The %CV values were also not different between breeds or over time. Thus, the data were combined, and mean overall within-animal variance and %CV values among all dogs were calculated (8.9 g/h·m2 and 18%, respectively; Table 1). Between-animal TEWL variation was considerably greater than within-animal measures, and statistically significant differences were found with time (Table 2). A statistically significant lower variance was observed in Beagles compared with hounds, but mean %CV values were not different between the two breeds. Despite the variance difference between breeds, an overall mean variance was calculated so that a descriptive appreciation of the overall variance of the TEWL measurement among dogs could be made.
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The average overall within-animal variance and %CV for HYDR values among all dogs were 11.7 g/h·m2 and 13%, respectively, and no statistical differences were observed between the breeds for either of these variables (Table 1). Between animals, both variances and %CV values were markedly higher than within animals. A statistically significant time effect (P < 0.05) of sample variances was observed when wk-3 data were compared with the other measurement times for Beagles but not for hounds. However, differences in sample variance between the breeds were not statistically significant. A statistically significant decrease was seen in the %CV at wk 3 compared with the other times in hounds, but generally, these values approximated 30% in both breeds at all time points measured (Table 2).
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DISCUSSION |
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In this study, a highly significant breed effect (P < 0.001) between Beagles and hounds was observed for both TEWL and HYDR measurements. Unlike an earlier study (4), animals were not specifically trained to stand quietly during the measurement period, although variability was similar to that earlier study. We found that allowing the dogs to lie on a padded floor combined with gentle manual restraint provided minimal movement and allowed measurements to be obtained in triplicate at 20-s intervals each. Body site was not varied in this study, and hair clipping of the inguinal region was performed 1 wk before any measurement.
A statistically significant time effect was also observed with higher HYDR values seen at wk 3 but not thereafter. It should be noted that the animals used in this study were fed different diets before their group assignments. A uniformly defined commercial diet was then fed beginning at wk 0 for the entire TEWL- and HYDR-measurement period. Consequently, the wk-3 HYDR values may have reflected the previous diets that the animals were fed. Future studies designed to determine effects of dietary modification on HYDR values should expect to feed dogs for >3 wk before reassessment. The 7- and 11-wk data support the existence of a new metabolic skin steady state during this time period. It is unknown whether such a new steady state is present at 4, 5, or 6 wk, because evaluations were not performed between 3 and 7 wk in this study.
Considerable between-dog variability existed for both TEWL and HYDR measurements. Nonetheless, clear differences between the two breeds evaluated in this study were found. However, it cannot be determined whether these differences were due to gender. The observed time effect for HYDR measurements is consistent with the concept that some period of time is needed to achieve a metabolic steady state in skin as a function of dietary modification. Finally, within-dog variations were less marked than between-dog variations under the conditions employed. It appears that both techniques are clinically useful, especially where pre- and posttreatment comparisons of individual dogs are performed or in studies where each animal serves as its own control. Finally, corneometry (HYDR) measurements appear to be more reliable and easier to perform than TEWL measurements. However, in view of new information on the variability of TEWL and HYDR measurements, it appears that both can serve as reasonably stable personal characteristics with which to investigate treatment effects of diet or medications on canine skin condition.
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FOOTNOTES |
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2 This work is supported in part by the Mark L. Morris Professorship in Clinical Nutrition at Texas A&M University and Nutro Pet Products, Inc.
4 Abbreviations used: CV, coefficient of variation; HYDR, skin hydration; TEWL, transepidermal water loss.
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LITERATURE CITED |
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1. Barel, A. O. & Clarys, P. (1995) Study of the stratum corneum barrier function by TEWL measurements. Skin Pharmacol. 8: 186–195.[Medline]
2. Pinnagoda, J. & Tupker, R. A. (1995) Measurement of transepidermal water loss. In: Nonivasive Methods and the Skin (Serup, J. & Jemec, G. B. E.), pp. 173–178. CRC Press, Boca Raton, FL.
3. Blichmann, C. W. & Serup, J. (1988) Assessment of skin moisture. Measurement of electrical conductance, capacitance and transepidermal water loss. Acta Derm. Venereol. 68: 284–290.[Medline]
4. Watson, A., Fray, T., Clarke, S., Yates, D. & Markwell, P. (2002) Reliable use of the ServoMed Evaporimter EP-2 to assess transepidermal water loss in the canine. J. Nutr. 132: 1661S–1664S.
5. Beco, L. & Fontaine, J. (2000) Coroneometry and transepidermal water loss measurements in canines: validation of techniques in healthy Beagle dogs [in French]. Ann. Med. Vet. 144: 329–333.
6. Nilsson, G. E. (1997) On the Measurement of Evaporative Water Loss. Methods and Clinical Applications. Thesis dissertation no. 48, Linkoping Medical University.
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